[NSRCA-discussion] OS-120AX questions/thoughts... yes again

[J N Hiller]

Jason I've been running a 120-AX for over two years. I have gotten by
(barely) without adding a pump by slightly restricting the muffler outlet
with a brass tube extension. The engine acts as if it doesn't have enough
muffler backpressure to the tank. I stripped the following explanation from
a 2-year old e-mail.

"Here is a photo of the exhaust restrictor I am trying on my 120-AX. The
13/32 (3/8 ID) brass tube is flared on one end to prevent it from coming out
of the exhaust hose coupler. I experienced lean running at full throttle on
an extended vertical running a 16-10 prop (APC). This is a lot of load and I
tried 10-20 fuel which helped but the transition was still too rich when the
high speed was adjusted rich enough to support an extended vertical. I
thought a little more fuel pressure (muffler back pressure) might help. I
also tried a 3/8 tube, which I felt was pulling the RPM down. This one
seamed to do the trick. I was able to adjust the high-speed needle in about
1/4 turn (now open 1 1/2). The transition on the bottom of the six sided
outside is now immediate. It will go lean when pulling vertical if the
throttle is advanced rapidly after pulling to vertical. If the throttle is
advanced slow enough for the RPM's to come up it works fine. I expect the
problem would not exist if running less prop load and more RPM's. I just
like the way the airplane flies with the 16-10."

My current flying style includes advancing the throttle as I begin pulling
vertical, achieving WOT about the time the airplane gets vertical. This
seams to work ok but if I go vertical before adding throttle it will go
lean.
I also experienced erratic running when the rear bearing was going bad. They
don't last very long. I am currently running the 140 RX-FI bearing as
suggested here about a year ago. OS # 29430010 It takes a while for the
noticeable seal drag to go away but after a season it still feels smooth as
new.

For what it's worth!
Jim Hiller


-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of J Shu
Sent: Sunday, September 20, 2009 1:43 PM
To: General pattern discussion
Subject: [NSRCA-discussion] OS-120AX questions/thoughts... yes again

Hi All,

I've posted in RCU about some issues I'm having with my OS-120AX and was
wondering what you might think?

Here's the the link: http://www.rcuniverse.com/forum/m_9110170/tm.htm

And what it says:

I'm hoping this thread will help answer some questions about the OS-120AX
that I am having.

I've got it in an 8lb 2oz, 110 sized pattern plane, stock muffler, 420cc
tank (14oz), OS-F plug, 3-line system (Vent, carb clunk
line, fill clunk line), 15x10 APC and for break-in, PowerMaster 10% with 29%
Caster... will switch to 15% with 16-18% Synthetic
after this gallon.

The tank is fairly close to inline with the carb, maybe 1/2" higher. On
neg/inverted pushes the motor seems to starve until I close
the throttle a little. On pos/upright pulls it seems ok, maybe a slight
richening. There is a nice smoke trail as after my first
flight I wasn't rich enough and the flight ended a bit early. I'm sure it
will run a bit easier after it's broken in, but has anyone
tried a foam clunk like the YS's use?

Any other tips? It has enough power with this set-up to go through the F-09
sequence, but about half way through it starts this
'cutting out' deal. It's just annoying to have to power back as you're
climbing to keep it running.

Also, what differences has anyone noticed flying the 15x10 to 16x8 in
testing.

Regards,
Jason
www.shulmanaviation.com
www.composite-arf.com

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